Turbulence heard in a headwind flowing by a sound sensitive system, such as the human ear is generated by two sources. The first is locally caused facial turbulence created alongside the human head of, say a bicycle rider because the air flow cannot remain attached past the high curvature of the cheekbone area. Instead, it separates in a turbid flow pattern which results in a noise spectrum varying among individual facial features as well as the relative wind speed, air density and humidity. The second type of turbulence is characteristic of the headwind itself being influenced by atmospheric disturbances: wind, thermals, etc.
Reduction of the first type of flow was addressed in the copending application. The aerodam is designed to attenuate facial turbulence of the bike rider by the use of an open sided filter mounted somewhat perpendicularly to the surface of the human head so that boundary layer stabilization is achieved at two zones: one in front of the aerodam, and the second behind in the wake around the ear canal. The forward zone tends to remain laminar up to the filter because the flow has been pressurized around the convexity of the cheekbone. The rear zone is called the flow envelope because it consists of a laminar wake overflow which encloses a slow flowing null zone caused by a filter. This mimics, in-situ, all those factors that contribute to turbulent decay, namely a diffusion and damping process. The above system assumes a steady headwind, a calm day.
When wearing a fixed aerodam of the type mentioned in U.S. Pat. No. 6,029,769, on a windy day the type-two atmospheric turbulence modulates the forward pressure zone as well as the flow envelope. The results sound like a random low frequency pulsation called infranoise. The atmospheric headwind is comprised of turbulence which can be measured as the time differential of the incoming wind shear per average frontal wind speed, or d/dt(curl v/v), or vorticity density. On a calm day the density is zero and the flow envelope pressure over the ear canal is steady. Scaled hydraulic experiments show that the average length and height of the envelope changes little as the flow speed changes. Air cannot flow perpendicularly to the surface so a turbulent vortex flattens out to flow parallel to the surface of the rider's head (Strasberg). This restricts the envelope pressure to vibrate unidirectionally in response to velocity changes in the wind shear patterns which cause changes in the Bernoulli pressure inside the flow envelope over the ear canal. So, not surprisingly, it is the differential change in pressure per increment of time (dp/dt) that is heard.
To protect the flow envelope pressure from vibrating, a commensurate change in the flow gradient of the dam is suggested, such as the height or density of the dam corresponding to the wind shear variation. In nature a narrow vertical tree or shaft of wheat leans over farther as the wind blows harder; the projected height H.sub.p of the shaft decreases and the relation of wind speed v to H.sub.p is negative, or dH.sub.p /dv&lt;0. In the fixed aerodam of the prior art the relation is zero or neutral because the matrix is rigid with respect to the wind, or dH.sub.p /dv=0. That leaves a third possibility: the higher the instantaneous wind, the higher the structure, dH.sub.p /dv&gt;0. This can occur if the structure were flexible and leaned into the wind. It is this structure that shall be studied.